1. Field of Invention
The present invention relates to a method of reducing peeling on wafer edges, laser marks and alignment marks. Additionally, the present invention relates to a method of easily reading wafer edges, laser marks and alignment marks.
2. Description of Related Art
FIG. 1 is a schematic, top view of a wafer with a laser mark. Before performing any processes on a wafer 102, laser mark trenches 106 are formed by a laser marking method on given locations outside defined die ranges 104 formed on the wafer 102. A laser mark 100 is composed of several laser mark trenches 106. The laser mark 100 is used for distinguishing lots of wafers 102.
FIG. 2 is a schematic, cross-sectional view of the area of the wafer with laser marks, showing how, in the later semiconductor processes, the wafer mark 202 on the wafer 200 is covered with a dielectric layer, a polysilicon layer and a metal layer in a stratified structure because of insulating requirements or layout of the wafer 200. The wafer 200 is covered with a polysilicon layer 204, a tungsten silicide layer 206 and a metal layer 208. A dielectric layer 210 is formed between the tungsten silicide layer 206 and the metal layer 208 for isolating conductive layers. In addition, a titanium nitride/titanium layer 212 used as an adhesion layer is formed on the dielectric layer 210 before forming the metal layer 208 in order to increase adhesion between the metal layer 208 and the dielectric layer 210. Finally, a thicker passivation layer 214 is further formed on the wafer 200 to ensure that structures on the wafer 200 are protected from damage.
The tungsten silicide layer 206 covering the laser mark 202 mentioned above is in direct contact with the polysilicon layer 204. The titanium nitride/the titanium layer 212 may also be deposited directly on the polysilicon layer 204. The adhesion between the tungsten silicide layer 206 and the polysilicon layer 204 or the titanium nitride layer 212 and the polysilicon layer 204 is bad, therefore, peeling easily occurs on the tungsten silicide layer 206 and the titanium nitride layer 212 after performing a later thermal process. The peeling has a bad effect on the processes.
In general, a chemical mechanical polishing process is introduced to planarize the metal layer 208 after forming the metal layer 208. The planarized surface of the metal layer 208 is quite reflective, which makes the laser mark 202 below the metal layer 208 difficult to distinguish. The thicker passivation layer 214 required in the last processes of semiconductors is formed on the wafer 200 to protect the wafer 200, but the laser mark 202 is difficult to distinguish because of the overly thick passivation layer 214.